1. Field of the Invention
The present invention relates to an apparatus for controlling a layer jump process of an optical drive, and particularly to an apparatus for controlling the layer jump process of a DVD drive.
2. Description of the Related Art
Generally, an optical disk can be classified into two categories according to the number of layers. That is, an optical disk can be a single layer disk such as a CD disk, a VCD disk or a DVD-5 disk, DVD-10 disk, or a dual layer disk such as a DVD-9 disk or a DVD-18 disk. When an optical drive reads the dual layer DVD disk, the pick up head of the optical drive moves its laser spot between the two layers. As a result, it is necessary to involve a layer jump process of the optical drive in order to move the laser spot from the initial layer to the target layer.
FIG. 1a to FIG. 1c are schematic diagrams of the pick up head in a conventional optical drive. The pick up head is composed of a lens 210 and a voice coil motor 220. The voice coil motor 220 is driven to move the lens 210 in the vertical direction. In FIG. 1a, the lens 210 is located in a natural equilibrium position B, as shown by the solid line. The lens 210 can be moved to the position L0 as shown by the dotted line in FIG. 1b, so that the laser spot is focused on layer 0 of the disk; otherwise, the position L1 as shown by line L1 in FIG. 1c, so that the laser spot is focused on layer 1 of the disk. As shown in FIG. 1b and FIG. 1c, a distance Do exists between the position L0 and the natural equilibrium position B, and a distance D1 exists between the position L1 and the natural equilibrium position B. The sum D=(Do+D1) is the layer distance of the disk.
The purpose of performing the layer jump process is to move the laser spot from the initial layer to the target layer. For example, the lens may move from the position L0 as shown in FIG. 1b to the position L1 as shown in FIG. 1c. Generally, the driving force involved in the layer jump process includes a control force to drive the voice coil motor to move the lens from the natural equilibrium position to the initial layer, a kick force to move the pick up head away from the initial layer, and a brake force to keep the pick up head in the target layer. The control force is always necessary when the lens is placed in the position L0 in order to keep the lens from moving back to the natural equilibrium position B. When the optical drive performs the layer jump process, the lens 210 moves out from the position L0 toward a linear controlled area of the position L1 by the kick force; then, the brake force is applied to slow down the lens 210, so that the lens 210 will stop in the linear controlled area when it reaches the position L1. Thus, the layer jump process is completed.
However, the aforementioned conventional method to control the layer jump process has its drawbacks as described below.
The dual layer disk, such as a DVD-9 disk or a DVD-18 disk, generally has a layer distance of 55±15 μm between its two layers, in which each layer has a linear controlled area of ±1 mm. It is obvious that the linear controlled area is relatively smaller, and the layer distance even has a larger variation. Therefore, if the optical drive estimates the position of the layer jump process from the natural equilibrium position B in each dual layer disk and applies the control force, the kick force and the brake force with the same intensity, it is possible that layer jump failure might occur in dual layer disks.
In addition, a dual layer disk may be irregularly manufactured or printed in the disk printing process, or be placed in an inaccurate position in the optical drive, so that the disk wobbles in rotating for the pick up head to read. If the disk wobbles, the disk layers sway up-and-down vertically in relation to the natural equilibrium position B of the lens. Therefore, the driving force irregularly increases or decreases in the layer jump process, which reduces the stability of the layer jump process and may cause failure.